Regulator pressure increases over time - why??

Problem: House pressure rises during the night from 40 PSI to 70 PSI. Is incoming pressure regulator failing?
Water meter pit is 175 feet from house. Pressure regulator is in ground next to meter pit. City water pressure in meter put approximately 100 PSI (according to city). Regulator is adjusted so that incoming house water pressure is about 40 PSI as measured on gauge in house. Pressure gauge in house is located on the house piping as it enters basement from the outside. Downstream of this gauge is check valve followed by thermal expansion tank. Also have pressure gauge screwed into hot water tank drain valve. Note that pressure readings on both gauges are approximately the same (within the accuracies of the gauges).
During the day when we are home and using the water, pressure is relatively constant from 38-42 PSI (within the accuracy of the gauge). However in the morning before we use the water, the pressure has risen overnight to 65-70 PSI.
Prior to leaving on a recent vacation, I shut valves in basement and released pressure on house piping. The pressure on the incoming water piping pressure gauge was 40 PSI and the pressure on the hot water tank gauge was about 0 PSI. When I returned from vacation, the pressure on the incoming water piping pressure gauge had risen to 70 PSI and the pressure on the hot water tank gauge was still at about 0 PSI.
Does this indicate a problem with the pressure regulator? If so, what is going wrong with the regulator mechanism? Should I replace the regulator?
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David wrote:

<SNIP>
Yes. Even the tiniest of leakage past the regulator will slowly increase downstream pressure. The valves are quite rugged and the leakage could simply be due to dirt accumulation. But, hey, if you have to remove the thing to clean it you might as well replace it.
Plan "B": Leave some faucet open to drip just enough to compensate.
Jim
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Plan "C": ignore it. Unless you're worried about your plumbing rupturing from 70PSI, it's not a problem. It'll instantly drop to 40PSI the moment you start using it.
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Not necessarily. A lot of places pump at night into storage containers cause it is cheaper. You need a gauge on the line side to make the determination.
My pressure regulator is set at 40 psi at the main incoming line.
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th>Not necessarily. A lot of places pump at night into storage containers cause

that wouldn't matter. that's the whole ppurpose of the regulator. No matter what they (water co.) do on the other side of the meter, his pressure won't rise above the 40p.s.i Of course it can drop though.
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snipped-for-privacy@aol.com (Playintennis5274) wrote in message

Unless the regulator is leaking slightly which is what it sounds like is happening.
Harry K
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i know that. . . i was commenting on the guy who said some crap about pumps.
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David wrote:

Based on the OP's thorough description I'd say it HAS to be an imperfect presure reducing valve.
My mind started running off in the direction of the pressure reducing valve being "perfect" but the incoming water was real cold, and the house warmer, which could cause a pressure increase as the water warmed up and expanded during the night. Then I went back and read that he had a thermal expansion tank in the system, which killed that bright idea.
The pressure reducing valve must be weeping more than just a tiny bit, because that thermal expansion tank (assuming it IS in proper shape) would have to accept quite a bit of water before it'd "bottom out" and permit the full line pressure.
Chances are a new valve, or a rebuild of the present one would reduce the weepage to the point where the expansion tank could easily handle it, certainly during an overnight idle period.
Jeff
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Some more data. I tried the following. Open house faucet. Pressue drops below 40 PSI, an expected result when using water. Then close whole house shut off valve. Pressure should rise within several seconds to line pressure comping out of regulator, right? Instead, I have to wait several MINUTES for pressure to rise. Does this further add to the evidence that the regulator shoudl be replaced?
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A pressure regulator is meant to hold (regulate) the downstream pressure when there is flow. When the system is shut down (faucet closed) the regulator shuts down also. If the system pressure rises to the upstream level it would mean (in my opinion) that there is some leakage past the shutoff seals. To be sure, one would need a schematic of the regulating valve to understand just how it works. Most regulators have a differential area so that the forces acting on the valves are such that if the downstream pressure rises above the controlled, or regulated pressure, the valve will close. MLD
MLD
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flow or no flow a regulator maintains the it's set for. go to home depot & in the sprinkler area they have little pressure guages for 10.00 BEFORE the pressure regulator take a reading (at your meter u should have some little threaded openings w/ little valve lever on the anti syphon valve) take a reading, then AFTER the regulator find a spot and take a reading. But i can guarentee you the regulator is slowly bypassing water. they're made of brass and soft the seat or the seal surface has cecome imperfect and is leaking.
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